The present invention relates to block copolymers intended to constitute an elastomeric matrix of a cross-linkable rubber composition with reduced hysteresis, to such a rubber composition which is used in the cross-linked state in a tire tread, to such a tire tread and to a tire comprising said tire tread which exhibits reduced rolling resistance.
The tire industry is constantly endeavoring to reduce the hysteresis of mixtures in order to limit fuel consumption and thus conserve the environment.
This reduction in hysteresis must be achieved while maintaining or even improving the processability of the mixtures. Numerous approaches for reducing hysteresis have already been investigated. Chain end functionalization has attracted particular attention.
Most of the suggested methods have involved seeking out adjacent functions on completion of polymerization which are capable of interacting with carbon black, for example contained in star polymers or tin-coupled polymers. European Patent Specification EP-A-709 235 may be mentioned by way of example. Other functions which interact with carbon black have also been attached to chain ends, such as 4,4xe2x80x2-bis(diethylaminobenzophenone), which is also known as DEAB, or other amine functions. Patent specifications FR-A-2 526 030 and U.S. Pat. No. 4,848,511 may be mentioned by way of example.
Some years ago, it became possible to use silica and research has been under way to find functions capable of interacting with this filler. Patent specification FR-A-2 740 778, which discloses for example functions comprising a silanol group, may be in particular be mentioned in this connection. Mention may also be made of specification U.S. Pat. No. 5,066,721, which discloses alkoxysilane or aryloxysilane functions, or also specification U.S. Pat. No. 3,244,664.
Most of these approaches, whether for carbon black or for silica, bring about a genuine reduction in hysteresis and an increased level of reinforcement in the corresponding compositions. Unfortunately, it is also generally the case that these improvements result in greater difficulty in processing these compositions.
Other means of reducing hysteresis which do not affect processing of the mixtures have thus been sought.
In particular, using polymers with a low hysteresis potential, in particular polyisoprene, has appeared to be a promising approach. However, directly using this type of polymer does not always provide a satisfactory compromise between dynamic modulus and hysteresis.
In order to overcome this disadvantage, the attempt has been made to use block copolymers comprising a polyisoprene block.
Block copolymers are generally composed of materials in segregated phases. Diblock polyisoprene/polystyrene copolymers, the synthesis of which has been comprehensively described in the literature, may be mentioned by way of example. These diblock copolymers are known to exhibit valuable impact resistance properties.
Block copolymers comprising polyisoprene and polybutadiene blocks (respectively abbreviated to IR and BR) have also been described in the literature.
Certain post-polymerization reactions convert these elastomers into thermoplastic materials. For example, when hydrogenating a triblock BR/IR/BR copolymer, the butadiene fraction forms a crystalline polyethylene, while the isoprene fraction gives rise to a rubbery ethylene/butylene type material.
Hydrochlorination of these materials may also impart crystalline properties thereto.
Diblock IR/SBR copolymers (polyisoprene/copolymer of styrene and butadiene) have been described in European Patent Specification EP-A-438 967 in relation to a reinforcing filler specifically comprising carbon black. The molecular weight of the IR block is preferably between 70,000 and 150,000 g/mol, while that of the SBR block is preferably between 220,000 and 240,000 g/mol. Furthermore, the ratio of the molecular weight of the IR block to that of the SBR block must be greater than 33% and may be as much as 300%.
The rubber compositions described in this document may be of a variable structure, which is lamellar when said ratio is of the order of 33%, and spherical when said ratio is of the order of 300%.
However, for all these values of said ratio ranging from 33% to 300%, it should be noted that the relatively high molecular weight of the IR block always results in marked segregation of the phases corresponding to the IR and SBR blocks respectively due to the high content of 1,4-linkages in the IR block.
IR/BR block copolymers have also been considered as compatibilizing agents for blends of polyisoprene and polybutadiene.
The paper by D. J. Zanzig, F. L. Magnus, W. L. Hsu, A. F. Halasa, M. E. Testa, Rubber Chemistry and Technology vol. 66, pp. 538-549 (1993), which mentions the use of IR/BR block copolymers containing 80% or 50% IR, may be mentioned in this connection. At these relative contents, the molecular weight of the IR block is always greater than or equal to 200,000 g/mol and, as a result, blocks of these copolymers also form segregated phases.
Mention may also be made in this connection of the paper by R. E. Cohen, A. R. Ramos, Macromolecules Vol. 12, issue 1, 131-134 (1979). In this paper, the diblock copolymers used comprise IR blocks of a molecular weight of 104,000 g/mol or of 133,000 g/mol. The relatively high weight of the IR blocks and BR blocks also gives rise to severe segregation of the phases associated with these two blocks.
The present invention relates to a copolymer with n blocks (n=2 or 3) intended to constitute an elastomeric matrix of a cross-linkable rubber composition with reduced hysteresis, each of said blocks comprising an xe2x80x9cessentially unsaturatedxe2x80x9d diene elastomer (i.e. having a molar content of units originating from conjugated dienes of greater than 15%), and one or each of said blocks which forms the chain end of said copolymer comprising a polyisoprene, when n=2 or n=3 respectively, which is such that the number-average molecular weight Mn1 of said or each polyisoprene end block is between about 2,500 and 20,000 g/mol, and such that the number-average molecular weight Mn2 of the block of said copolymer which is other than said or each polyisoprene end block is between about 80,000 g/mol and 350,000 g/mol, permits significant optimization of the results for hysteresis reduction and processability for said rubber composition.
The subject of the invention also relates to cross-linkable or cross-linked rubber compositions comprising an elastomeric matrix containing the block copolymer according to the invention.
The subject of the invention is also tires and, in particular the treads, comprising a rubber composition according to the invention, to achieve reduced hysteresis.